A suitable device is needed for the direct measurement of oxygen partial pressure (PO2) in blood and tissue for both clinical and research applications. Methods currently available for measuring P02 lack convenience, reliability, speed, and relevance to many situations of interest. Efforts to developed electrical sensors have not been successful. It is desirable to have a very small P02 sensor which can be inserted into a blood vessel of tissue with little disturbance, and which will provide instantaneous and current P02 monitoring for either short or extended periods of time. A fiber optic sensor is ideal for this application, with the advantages, for physiological use, of very small size and flexibility, safety, and low cost. A P02 sensor has been developed, based upon the principle of fluorescence quenching by oxygen. In the previous year the feasibility of the sensor was demonstrated and its performance evaluated with in vitro and animal tests. Current work is oriented toward converting the sensor to a needle form for experimental use, improving the instrumentation and probe construction, and solving some remaining problems with the sensor system to increase its utility.